Hi Richard, thank you very much for the explanations. On Sun, Mar 20, 2016 at 02:47:18PM -0400, Richard Eisenberg wrote:
GHC 8 does implicit quantification a little differently than previous versions. Previously, writing `=>` in a type meant that GHC looked through the type for free variables and put an implicit `forall` for those variables out front. GHC 8 doesn't do this, explaining the first change.
That makes sense, thanks.
As for the second change, GHC 7.10 should not have accepted the old program, as it was incorrect. The problem is that GHC must infer types for f' and g'. These types are constrained (they have `Applicative f =>`). They have no type signature, nor are they declared using function syntax. Thus, the monomorphism restriction applies, causing chaos. I bet enabling -XNoMonomorphismRestriction would allow you to undo the second change.
According to Travis the old program compiled on 7.8 and 7.10. It certainly compiled on 7.6 on my local machine. When was the change that made it break introduced (and what was that change exactly)? Tom